The present invention relates to a method of measuring distortion by aberration of a lens and, more particularly, to a method and apparatus for measuring a positional shift and distortion of an image point by aberration of a lens of a projection optical system used for reduction projection exposure.
In the exposure (lithography) process for manufacturing a semiconductor element, liquid crystal display element, or thin-film magnetic head, a reduction projection exposure apparatus is used, which, using a reticle having a predetermined magnification as a mask (photomask), projects a pattern reduced to the original size through a projection optical system and exposes it. In this reduction projection exposure apparatus, to accurately form the pattern image of the reticle on the resist of the substrate and expose it, aberrations in the projection optical system must be made as small as possible.
Of aberrations of the projection optical system, an aspherical aberration such as coma (also called xe2x80x9ccomatic aberrationxe2x80x9d) due to the magnification difference between various annular zones of a lens is inspected using a reticle having an inspection pattern, e.g., a line-and-space pattern having a pitch about twice the exposure wavelength. More specifically, the inspection pattern is exposed on a substrate coated with a resist. The asymmetry of the resist image of the inspection pattern formed by a substrate development process is measured using an SEM (Scanning Electron Microscope). On the basis of the measurement result, the coma of the projection optical system is measured. Note that Japanese Patent Laid-Open Nos. 6-117831 and 8-78309 are referred to for aberration measurement and distortion measurement.
As described above, in the conventional method of quantitatively measuring the coma of a projection optical system, an inspection pattern is exposed using a reticle having several line-and-space charts arrayed at a pitch about twice the exposure wavelength, the line width is measured using a length-measuring SEM, and the coma amount is calculated on the basis of the line width difference between the two ends of the pattern. In this conventional method, however, although the coma amount can be estimated, the positional shift of an image point due to the coma cannot be measured.
As for distortion of an image by a lens as well, the distortion amount can be measured. However, the designed pitch of the distortion check mark is different from that of an actual device. For this reason, in a lens of a projection optical system having coma, the positional shift as a measurement value is different from that in device exposure.
In measuring distortion including a positional shift due to the coma of the projection optical system lens of a reduction projection exposure apparatus, the positional shift due to the line width difference of the inspection pattern cannot be measured. Hence, as one of important objects, automatic, high-speed, and easy distortion measurement need be realized.
It is an object of the present invention to provide a method and apparatus for measuring a positional shift/distortion by aberration, which allow automatic measurement of positional shift components of an image point in lens aberration, including both distortion and a positional shift of the image point due to coma.
In order to achieve the above object, according to the present invention, there is provided a distortion measuring method comprising the steps of forming a mask having at least a first diffraction grating pattern having an array of a plurality of large patterns and a second diffraction grating pattern having arrays of a plurality of micropatterns spaced apart from the first diffraction grating pattern by a predetermined interval, the plurality of micropatterns being arrayed in a direction perpendicular to an array direction of the second diffraction grating pattern at a predetermined pitch, projecting at least the first and second diffraction grating patterns formed on the mask on a photosensitive substrate through a lens, and scanning the photosensitive substrate using coherent light having a diffractable wavelength and measuring an interval between at least the first and second diffraction grating patterns, thereby measuring distortion including a positional shift component of an image point by aberration of the lens.